The invention relates to a "fire-safe" valve which avoids dangerous pressure build-up in the valve chamber by establishing communication between the valve chamber and the flow passage through the valve when the environment in the valve chamber reaches a preselected temperature. More specifically, the invention relates to a valve of the type having an expansible gate assembly wherein the gate assembly is provided with means for providing communication between the valve chamber and the upstream flow passage in the event the environment within the valve chamber reaches a preselected temperature condition and the valve retains the capability of providing an upstream seal in the event of subsequent leakage from the valve chamber.
It is a recognized problem that sometimes when a gate valve is exposed to a fire condition wherein the valve chamber is sealed off in the closed or open condition of the valve, the resulting high temperatures can create abnormally high pressures in the valve chamber which results in the destruction of the valve as by shearing of the bonnet bolts or fracture of the valve body. There have been attempts to deal with this problem. For example, in U.S. Pat. No. 2,670,752, there is disclosed a gate valve having check valves mounted in the gate assembly for relieving excessive pressure in the valve housing. This device utilizes spring-biased ball checks which must seat and seal during normal operation of the gate valve. Leakage past the ball checks sometimes occurs due to particle contamination of the effluent which hinders the function of the springs or the check valve seats. In U.S. Pat. No. 4,393,889 there is disclosed a "fire-safe" gate valve structure which utilizes a relief plug installed in a port in one of the valve seat rings and exposed to the environment of the valve chamber. The relief plug is adapted to evacuate the port into the upstream flow passage upon the fluid environment in the valve chamber reaching a preselected pressure condition or when a preselected pressure differential exists between the valve chamber and flow passage. However, once the plug has been expelled there is no upstream seal for the valve. Also, in applications where it is required that the valve seat rings be interchangeable and this type of seat ring with relief plug is employed for both the upstream and downstream sides of the gate assembly, once the preselected temperature is reached, the valve is never again able to close the flowline through the valve.
It is therefore, an object of the invention to provide an improved valve structure which enables the valve in operation to withstand abnormally high temperature situations by allowing pressure within the valve chamber to be relieved by establishing communication between the valve chamber and the upstream flow passage when the environment in the chamber reaches a preselected temperature and after the valve chamber has been relieved, is still capable of providing an upstream seal if there is a failure of the downstream seal or leakage from the valve chamber.